Increasing demand for a range of high-bandwidth communications services is driving a need for high-capacity access networks to provide those services. Optical access networks can deliver the high bandwidths now required. An optical access network typically has apparatus called an Optical Line Terminal (OLT) at a Central Office node. The OLT serves a plurality of optical terminals, called Optical Network Units (ONU). ONUs can be deployed at subscriber premises, at kerbside cabinets, or at other remote locations, depending on the access network architecture. A Passive Optical Network is a type of optical access network with limited, or no, power requirements in the optical path between the Central Office and ONUs. There are various types of passive optical network which differ in how the resources of the fibre are shared among ONUs. In a Time Division Multiplexing Passive Optical Network (TDM-PON), the resources of the fibre are shared on a time-divided basis among ONUs. Traffic in the downstream direction is broadcast by the OLT to all ONUs, with each ONU extracting traffic destined for itself. Each ONU served by the OLT is allocated time slots in which it can transmit data to the OLT. The time slots can occur at irregular intervals and can have irregular durations. In a Wavelength Division Multiplexed Passive Optical Network (WDM-PON), each ONU is allocated a different wavelength channel, called a lambda, for communication between the OLT and that ONU.
Techniques for reducing the energy consumption of optical access networks have been proposed. In TDM-PONs, energy is consumed by transceivers to keep the link between the ONU and OLT alive, regardless of traffic. It has been proposed to power off the ONU transceiver in a TDM-PON at times of no traffic to save energy.
One proposal is that an optical network unit (ONU) can autonomously enter a low-power state during times of inactivity. This means that an ONU decides for itself, without external control, when to enter a lower power state. Another proposal is that an external entity, such as an OLT, authorises an ONU to enter a lower power state at the discretion of the ONU. When the ONU decides to sleep, it signals to the OLT so the OLT can distinguish between the ONU being asleep and the ONU being at fault. One proposal for ITU-T G.987.3 is for two non-autonomous reduced-power modes referred to as cyclic sleep and doze mode. Cyclic sleep refers to the controlled powering off of the ONU transceiver during short time intervals. Doze mode refers to the controlled powering off of the ONU transmitter, while keeping the ONU receiver powered up and active.
Energy management typically presents a trade off between ONU energy savings and service quality (or QoS), and is something that is managed in a closed manner between the OLT and ONUs.